Conventionally, as disclosed in Japanese Laid-Open Patent Application No. 4-121260, a brake force control apparatus is known which generates an increased brake fluid pressure as compared to that generated during a normal brake operation when a brake pedal is depressed at a speed exceeding a predetermined speed. A driver of a vehicle operates a brake pedal at a high speed when a rapid increase in a brake force is desired. According to the above-mentioned brake force control apparatus, when such a brake operation (hereinafter referred to as an emergency brake operation) is performed, it is possible to generate a brake force which appropriately responds to a request of the driver by generating an increased brake fluid pressure as compared to that generated during the normal brake operation.
A brake force requested by the driver after start of the emergency brake operation is not always uniform. That is, when the brake pedal is further depressed after the start of the emergency brake operation, it can be determined that the driver intends to increase the brake force. On the other hand, when an operation of the brake pedal is maintained constant or decreased, it can be determined that the driver intends to maintain or decrease the brake force, respectively.
Accordingly, in order to generate a brake force to which the driver's intention is accurately reflected after the emergency brake operation is started, it is appropriate to increase or decrease the brake fluid pressure in accordance with a state of operation of the brake pedal after the brake fluid pressure is increased as compared to that generated in the normal operation. Such an ability can be achieved by, for example, providing an additional function which attempts to increase the brake fluid pressure when the amount of operation of the brake pedal is increased, maintain the brake fluid pressure when the amount of operation is maintained, and decrease the brake fluid pressure when the amount of operation is decreased.
However, it is not necessarily easy to increase or decrease the brake fluid pressure supplied to wheel cylinders when the brake fluid pressure is relatively high as compared to a case of a normal operation. For this reason, the brake fluid pressure intended by the driver may not be generated if the brake fluid pressure is controlled by using the above-mentioned technique.